Re: Dish questions for 2.4GHz and 24GHz

Several local hams are using DH Satellite 11 series 2 foot dishes on 24 GHz
during during contests. The data sheet states that the surface is accurate
to 0.02" RMS so the loss due to surface irregularities at 24 GHz should be
under 2 dB.
73,
John
KD6OZH
----- Original Message -----
From: "Tom Clark" <tac@clark.net>
To: "Don Woodward" <dbwoodw@abraxis.com>; "amsat-bb" <amsat-bb@AMSAT.Org>
Sent: Friday, 21 September 2001 02:44 UTC
Subject: RE: [amsat-bb] Dish questions for 2.4GHz and 24GHz
>
>
> > -----Original Message-----
> > From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> > Behalf Of Don Woodward
> > Sent: Thu, Sep 20, 2001 22:03
> > To: amsat-bb
> > Subject: [amsat-bb] Dish questions for 2.4GHz and 24GHz
> >
> >
> > Would the dish below from DH Satellite be suitable for 2.4GHz now and
then
> > be adaptable to 24GHz later provided substitution of two correct
> > feed mounts
> > and feeds?
> >
> > Don Woodward
> > KD4APP
> >
>
> They don't say anything about the quality of the dish surface. To have
good
> efficiency, a dish
> should have an RMS surface accuracy (both the general shape of the
parabola,
> and the roughness of the surface) "smooth" at levels of 1/20 wavelength.
> Since 24 GHz = 1.3 cm, this means that the quality needs to be at the 600
> micron level.
>
> And a 1.2 m dish operating at 1.3 cm has 120/13 = 92 wavelengths in its
> aperture. Therefore the beamwidth (full width at half max) will be ~1/92
> radians (the fact that the aperture is circular actually ~1.22/92 = .013
> radians = .76 degrees = 45 arc minutes. To use such a dish you will need
to
> somehow point it to a fraction of the size of the beam, i.e. something in
> the range of tens of arc minutes. Just to put this in perspective, the
hour
> hand of an ordinary 12 hour analog clock moves thru 360/12 = 30 degrees
per
> hour. at 24 GHz, you would have to be able to point this dish to an
accuracy
> better than the angle that the hour hand moves in a minute of time in two
> dimensions.
>
> So the dish might work at 24 GHz, but I'm not sure that you would find it
> very useful!
>
> 73 de Tom, W3IWI
>
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